In an injection-molding process, a plastified mass of a synthetic resin material is introduced under pressure (injected) into a closed mold forming a mold cavity which is filled by the flowable mass of synthetic resin. The latter is customarily at a temperature above its plastic-flow temperature and generally above its melting point.
Upon cooling, the mold is opened and the article is ejected.
Injection molding systems of conventional types operate under a variety of modes.
For example, a predetermined quantity or dose of homogeneously melted or plastfied synthetic-resin material is injected under high pressure into a mold cavity which is complementary to the configuration of the article to be produced, i.e. is a negative of this article. The mold cavity is generally defined between two or more mold parts which are separate to allow release of the molded article and which are cooled.
The dose or quantity of the synthetic-resin material is a function of the volume of the mold cavity and is generally precisely dimensioned by introducing the necessary quantity of the material from an injection nozzle via an injection cylinder communicating with the mold cavity.
The injection cylinder is customarily provided with an injection piston which has a stroke sufficient to advance the mold material from the dosing chamber into the mold cavity.
As soon as the injection-molded material hardens by heat abstraction by the mold wall, i.e. by heat transfer to the latter, the mold cavity is opened and the molded article is ejected from the mold cavity. The mold is then closed and the process begins anew.
The cooling of an injection-molded article by heat transfer from the mold bodies through the mold wall of the cavity has been found to be relatively slow. Since the ejection of the injection-molded article from the mold cavity with the usual ejectors can only be effected when the injection molded article is hardened to its core, the heat transfer required for cooling the article must continue from the point of mold filling until the cooling has progressed sufficiently to allow such ejection.
Should the ejection force be applied to the molded article as long as the core is still soft, there is the danger that the article will be deformed in the region of the applied force, thereby rendering the article unusable and requiring its discard.
The working cycle of the injection molding machine is thus a function of the cooling time for the individual or successively produced articles.
Especially when thick-walled injection molded articles are to be produced, the cooling time per article can reduce the cycling time of the machine so that the capacity thereof is diminished below economical levels.
It is known to attempt to increase the cycling time of the machine by a forced cooling of the injection molded articles. However, the problem with this techniquie is that high-speed cooling of the injection-molded article, especially when it is large or formed from complex parts, can result in inhomogeneitus, stress regions and distortion zones because of thermal phenomena. The resulting reduction in the quality of the product causes the number of rejects to be large.
In fact, in practice, complicated-contour molded articles make use of heated molds or so-called hot-runner molds to insure the homogeneity of the injection-molded articles.
A process which involves the heating of the mold, the cooling of the mold and the subsequent reheating of the mold for the next cycle, increases the energy cost beyond economical or reasonable expectations and does not significantly reduce the cycling time of the machine.